The Home Computer Course 06

An ©RBIS Pubication

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^ardware

Domestic Science You may be surprised to
learn how many household appliances
contain a microprocessor

Atari 400 & 800 These micros are leaders
in the games market

106

109

Charting The Course The best way of
planning well-structured programs

Two's Company We discover how
computers perform multiplication

insights

Computing Careers Entering the
professional world of computing

The Missing Link Modems allow micros to
communicate over telephone lines

Flat Spin Disk drives offer high-speed
information retrieval and storage

104
119

101
108
114

Basic Programming

>

Braving The Elements We introduce
subscripted variables

116

passwords To Computing

Digital Dialogue The two-way flow of
information between the computer and its
peripherals

112

t

1 Pioneers in Computing

9

Sir Clive Sinclair The man who has made
computers accessible to almost everyone

120

Home Computer News A review of
personal computers as seen at London's
Barbican Centre

INSIDE
BACK
COVER

Next Week

• We look at the Dragon 32, a
home computer successfully
marketed by the Welsh
Development Agency, which
offers good facilities and
excellent value for money

• From sand to circuit. We
reveal the intricacies of chip
manufacture

• In recent years the field of
medKine has benefited
considerably from the
introduction of microprocessors

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Insights

Computing Careers _

computer professional acquires skiii by worlcing initially as a
technician, and progressing up through the ranks

Gentle Giants

Large commercial computers
like this one (known as
'mainframes' to differentiate
them from mini- and
microcomputers), require a
team of highly trained operators
to keep them running at peak
efficiency Machines of this size
are capable of running
hundreds of programs
simultaneously and serving
thousands of users anywhere in
the world by means of
telephone lines, microwave
links and communications
satellites. A computer room will
often contain a number of such
machines, each communicating
with the other

The increasing use of computers at home and in
schools is producing many gifted programmers —
people who might otherwise never have
considered the possibility of a career in
computing. But the harsh truth is that, as always,
a little learning is a dangerous thing — especially,
it appears, if that little learning is of the basic
language.

It is important to understand that the
requirements of a professional programmer are

fundamentally different from those of a home
user, and that many of the attributes are not
transferable.

For the school-leaver with a profound interest
in computers, a college course on the subject, or
direct career entry to computing, seems an
obvious choice. Many colleges and universities
offer degree courses with a computer

qualification at the end and successful students
are likely to find themselves able to choose from a
variety of job offers. Unemployment in the
computing industry has been limited to lower-
level computer staff — largely programmers and
operators — and the demand for engineers,
systems analysts and designers continues
unabated.

One option becoming increasingly available is
to teach computing at school. Until now,
computing as a subject in its own right has been
the preserve of universities and colleges.
Education is desperately short of trained
computer personnel, and such a career would
undoubtedly be very rewarding.

There are perhaps six main levels of hierarchy
in the computer industry. The lowest grade may
be described as 'skilled user.' This category
includes workers who have learned to operate
computers in particular tasks, such as word-
processing or accountancy. Often these skills are
picked up as a sub-set of skills to other
occupations — e.g. secretarial or office
administration — but they also include computer
industry functions such as terminal operator,
card-punch (data) operator and the like. These
jobs require a basic set of school or college
qualifications, and the ability to think clearly.
Skills such as keyboard operation are normally
taught on the job.

Next step up is the computer operator. Though
the computers used in industry are quite different
in appearance and feel fi*om home computers, they
are based on the same principles, so some
familiarity is useful. Operators soon come to
understand the fundamentals of how computers
work, and so becoming an operator is a good
springboard to becoming a programmer. Bear in
mind, though, that the work can be quite
demanding physically. Most large installations,
for example, are in operation for 168 hours a
week, and need to be manned for all that time.

To become a programmer, the main attributes
one needs include a clear, methodical mind and
an ability to concentrate on minute detail. It takes
a very special type of aptitude to make a skilled
programmer, and while normal entry qualific-
ations 2ire a degree or senior school-leaving exam
passes, natural ability to work logically often
counts for more. Programmers do enter the
industry without formal qualifications, and it is

this opening that attracts many parents, hopeful
for their sons' or daughters' programming abilities.

THE HOME COMPUTER COURSE 101

The Choice For A Lifetime

Analysts

Before starting on any job, it is
as well to look closely at the
objectives and the resources available.
The Systems Analyst has
the task of interviewing users,
to determine their needs, to
match these needs with
resources, and suggest a
method of solving the problem.
In order to evolve a system of
working for other people, the
Analyst must be a logical thinker with good

communications skills and a spark of creativity.
He is often the DP department's salesperson ,
so must always make a favourable impression
on his 'customers' — the computer
users in the company.

Programmers

The Programmer takes the
broad strategy worked out by
the Analyst and converts
it, first into a tactical plan,
breaking the job down into
manageable segments, and then
into code that the computer can
recognise and interpret.
Applications Programmers
are concerned with writing
programs to do specific jobs, while

Systems Programmers are more involved
with the overall

nil

jUjll

performance of the
data processing system.
Applications Programmers
tend to work in isolation,
even though they may be
part of a project team.
For them, the ability to

concentrate attention on the
task in hand is really important. Systems
Programmers need that too, but also a calm
outlook. 'If you can keep your head when
all about you are losing theirs . . .' then
perhaps you have the makings
of a Systems Programmer.

I

W4

Operators in smaller

nstallations are often called on
to help programmers and
engineers diagnose faults, as
well as simply running the job
at hand. Most important,
though, is a thprough
knowledge of the program's
operating method. 'User-
friendly' software makes the
operator's job easier, and a well
prompted' program can be run
by relatively inexperienced staff
with little loss of efficiency.

Development Engineers

Though the time may come
when computers themselves develop the

new generation of
machines, it's in the
brain of the Development
Engineer that this process
of innovation takes place
now. The development
engineer is part scientist,

part technician. It is his job to take advantage of new
discoveries and theoretical developments to improve
and enhance the performance of a given piece of equipment.
Doctorates abound in this field where even the least well
qualified is likely to have spent five or more years at university.

Like any other part of
a modem corporation,
the computer department
is organised along
hierarchical lines. At its
head is the Data
Processing Manager,
who is responsible for all
the many and varied tasks
that fall under the main
heading of information
processing.

All computer professionals
are firstly technicians, and
acquire management skills as
they progress up through the
ranks. The three main areas
of specialisation are computer
operations, programming,
and systems analysis, and
there is an element of mobility
between specialisations in the
promotion path.

In common with the other
professions, it is
worth entering the field
as well qualified as possible.
While it may not appear to make too much difference at the
beginning, a less qualified person will soon find the path
barred. It's much more difficult to get a university degree
while doing a full time job! Additionally, organisations like

the British Computer Society

-<

Field Engineers

Often, the only chance an Operator
has to relax is when
the computer develops
a fault, and a Field
Engineer has to be
called in to fix it.
Given the modern
computer's ability to
diagnose its own
failings, and the
almost universal
adoption of modular
construction, the
engineer's job has

become somewhat simplified,
but a field engineer must still be
competent in digital electronics. He must
also be a skilled mechanic, capable of
working to finer tolerances than the average
watchmaker. To enter the field, a degree level
qualification is usually required.

Operators

Physically, the most demanding
of all jobs in the industry is operating a
large multi- programming/multi-user computer.

But as well as walking

miles in a shift with
disk packs, tapes or
boxes of paper, the
operator must be
fully conversant with the
computer's operating system,
and with the relative
importance of the jobs
being run on the
machine at any one

now offer professionally-
recognised qualifications,
usually by examination,
and for an aspiring
programmer or analyst these '
are a good indicator of
standing within the industry.

time. A Senior Operator will be called on
to make decisions affecting the work of
many other parts of the company's
business by allowing or
denying access to
the computer system.

Least demanding of all,
intellectually, is the Data Entry
Operator's job. The skills
required here are much
the same as those
needed by a copy typist
— speed and accuracy.

At worst the task is i^;^
boring and repetitious,

but in many small installations this Is
offset by the opportunity to
become involved in other
aspects of the computer
department's activities.

102 THE HOME COMPUTER COURSE

As we mentioned earlier, an understanding of
BASIC is not necessarily an open door to the
computing industry. Although it is a popular
language on home computers, most professionals
regard it as badly structured and consider that it
encourages bad programming habits and sloppy
thinking. This is a real problem as most children
with home computing experience are likely to
have learned basic rather than one of the better
structured languages such as logo or comal.

Several universities and colleges that run
courses in computing now express a preference
for entrants who have not learned basic as they
feel that the language forms habits that are hard to
break.

Despite this problem, many youngsters are
finding a way to make their basic programming
skills pay off. Many are writing games in basic
that appeal to other youngsters, and software
companies are anxious to get hold of games that
are so directly targetted to an adolescent mind.
Some of the 'whizz kids' featured in newspapers
as earning vast sums at a young age write only in
BASIC, and have no real understanding of
computing. Others are genuine phenomena,
writing in Assembly Language (the low-level
language that controls a microprocessor's
machine code very efficiently) and set for a very
bright future indeed. Newspaper reporters are
seldom qualified to distinguish between the two,
and such reports can lead parents to think that
their computer-obsessed offspring is ready to
enter the big money. It is possible, but unlikely.

Inside The Business

In the computer industry proper, programmers
are split into two groups: applications
programmers and systems programmers.
Applications programmers write programs to
carry out a specific task. Systems programmers
are 'housekeepers', writing programs to keep the
computer system in order — to detect faults, for
example. The applications programmer is likely
to meet people outside the computer room —
clients — and is likely to work as part of a team
developing programs for a specific task. The
systems programmers are more specialist, and
tend to work alone. They are talking directly to the
machine's 'intelligence'.

But at this point, the computer industry draws
an artificial dividing line, beyond which it denies
access to all but the brightest programmers and
the best-qualified university graduates. This is the
realm that belongs to the systems analysts and
designers.

Systems analysts consider a problem and then
decide how a computer can help to solve it. For
example: an oil company discovers a new deposit
under the sea bed. They have measured the extent
of the deposit and found that the quality of oil
varies widely. The oil company has to decide
whether or not to invest the billions of dollars
necessary to exploit the oil field. This decision will

be based on projections about the state of the
international oil market for the life of the field
(say 20 years) and the company must decide
which part of the field to drill first. Because the
investment is so vast, the oil company hands the
problem to its computer people for analysis. The
analyst considers the problem, consults
economists, oil marketing experts, geologists and
other specialists and over a long period of time
constructs a computer 'model' of the oil field.

The oil company executives can then play
'what if?' with this model, discovering how
various decisions about price, refining techniques
and market approaches would affect overall
performance, and they are provided with all the
information they need to make their final
decisions about how best to exploit their field.

There are several other important roles in the
computer industry, although few are as highly
regarded as the systems analyst. Perhaps the
exception is hardware design. There are openings
for electronics engineers at all levels from high
street repair centres to research departments, but
the areas of product development and pure
research are only open to those with the highest
electronic engineering qualifications.

Many analysts and designers of both machines
and software move on to managerial and
consultancy positions, but these titles often
indicate only that the individual is working in a
more powerful role, very often self-employed.
The work content of the job often remains the
same.

On any typical day there is a massive shortfall
in skilled computer personnel — some put it as
high as 20,000 or more in this country alone — ■
and at the same time a huge pool of unemployed,
many of them graduates of universities and
polytechnics. This obvious skills mis-match is a
source of worry to educationalists and
industrialists alike, and serious steps are being
taken to rectify the situation, including re-training
programmes for those qualified in other fields and
a much wider variety of opportunities to learn at
primary, secondary and tertiary levels.

Several governments, Britain's in particular,
consider that microelectronics may provide an
answer to some of the short-term unemployment
problems. The Youth Training Scheme, which
aims to provide 'on the job' training and work
experience for unemployed school leavers, now
offers 4,500 places at Information Technology
Centres in Britain. At these centres, young school
leavers learn about various aspects of
microcomputing while receiving a training
allowance equal to unemployment benefit. Other
projects within the scheme offer some computer
familiarisation to those who fell through the net at
school (either because they left school before the
computer arrived, or because they weren't
'selected' to use it) and also improve their
prospects of finding a job, because for those who
leave school without any computer familiarity or
literacy, employment prospects can seem grim.

J

David Simmonds

David Simmonds, 17, earned
himself £10,000 during his
summer holidays.

He's a programming wizard
who writes programs for
Commodore (makers of the PET
and Vic computers). Unlike
many teenage boys, David
writes 'serious' software that
has commercial applications
and he is expecting to find a
lucrative niche in the computer
industry when he has finished
studying.

David started playing with a
computer his father brought
home from work, but he quickly
abandoned game-playing and
got down to discovering how to
program. Initially David had
some of his programs
published in Commodore's user
magazine and slowly but surely
he began to sell copies of his
programs, for a few pounds

Commodore eventually took
notice and David persuaded
them to let him show them
what he could do. The result
was his first serious
programming assignment

Eugene Evans

Eugene Evans is 17 years old
and his earnings are reported to
be £40,000 a year!

Eugene is one of the many
whizz kids now springing up in
computer programming and he
is helping to keep his
employers. Imagine Software of
Liverpool, among the top
computer game producers in
the country.

The high earnings made by
these programming wizards
usually take the form of
royalties on the sales of games
(rather like authors' book
royalties) and teenage boys are
best suited to develop games
that will appeal to other teenage
boys — the main market for
computer games

THE HOME COMPUTER COURSE 103

Charting The Course

Tlieconsci^^ and well-

organised programs

The Flow Of Information

The real purpose of a flow
diagram is to indicate in a
simple, concise manner, the
flow of information and control
through a computer program.

Most important are the 'test'
points, where control passes to
a point other than the next in
sequence. A simple graphical
representation of this passage
of control is much easier to
grasp than a similar statement
written out — a picture really
can be worth a thousand
words!

The 'TEST' symbol can be
represented by either a
flattened hexagon, as shown
here, or by an elongated
diamond shape

TERMINATOR

This symbol is used to
indicate the beginning
or end of a sub-
program

INPUT/OUTPUT

Any data entry or display
(I/O) is represented by
the parallelogram
symbol

PROCESS

This represents any process
that performs an action
internal to the program

TEST

This symbol is used when a
choice is made between the
alternative flow paths,
TRUE or FALSE

CO
UJ

-z

O

>

A problem can be represented in a simple
pictorial way by drawing diagrams to show the
steps required in processing, and the flow paths or
routes connecting them. These 'flowcharts' are
useful as a means of understanding a problem,
and in working out its solution.

Each box symbol in a flowchart represents a
process or action, and the lines that connect these
action boxes depict the possible paths through
them. Traffic flow' is one-way, so arrows are used
to indicate direction, which is normally top to
bottom, and left to right across the diagram.

Whenever a choice is to be made, a hexagonal
or diamond shaped 'decision box' is used.
Control flows in by one path, as before, but may
pass out in one of two directions, depending on
the result of the test in question. If the test is to
determine whether a single process is to be
performed or not, then only one of the exit paths
will contain a 'process box'. Here is an example of
a test to decide whether or not to branch to a sub-
routine:

False

O

True

X = Y ? N ^

>

SUBROUTINE

T

120IFX = YTHENGOSUB300

The decision box is also used to indicate the test
that terminates a loop. In the example given
below, control is returned to the start of a program
if there is a positive reply to the question 'AGAIN?':

: C

90 REM** START OF GAME**

100

800

104 THE HOME COMPUTER COURSE

810 PRINT "AGAIN? (Y/N)";
820 INPUT R$

830IFR$ = T'THEN GOTO 100
840 END

We may wish to make a decision that will result in
one of two distinctly different courses of action
being followed. In the example shown below, we
compare a player's game score to the highest
previous score:

THISSCORE >
HIGHSCORE?

Yes

OUTPUT
'HARD LUCK'

HIGHSCORE
THISSCORE

I

OUTPUT
'CONGRATULATIONS'

I

OUTPUT
HIGHSCORE

7

1200 IF THISSCORE > HIGHSCORE THEN GOTO
1230

1210 PRINT "HARD LUCK. YOU HAVE TO BEAT";
1220 GOTO 1250

1230 LET HIGHSCORE = THISSCORE
1240 PRINT "CONGRATULATIONS! A NEW HIGH
OF";

1250 PRINT HIGHSCORE

Note that the value of HIGHSCORE is printed in
both events, and that the two possible flow paths
rejoin in the process to become the single entry to
this output operation.

All decisions are taken as a result of tests
similar to this, which deliver a positive or
negative, a True or a False result. As you can see,
this purely binary decision-making process denies
the possibility of a 'maybe' answer. You can use
whatever terms you wish, but don't forget to label
the two exit paths accordingly!

All programming languages have an inherent
decision statement which, if the True condition is
satisfied, cause a conditional branch, but which
drops control through to the next statement if the
result is False. In the case of a dialect of basic that
allows only a simple IF-THEN, we must mimic the
conditional branch by means of a GOTO statement,
as in line 1200 of the last example. The statement
in line 1210 will only be executed if the result of
the test in line 1200 is False.

But what about the second use of GOTO in line
1220? As you can see, the use of GOTO at the end
of the test, to solve the problem of the destination
of the conditional branch, has forced us to use this
method to 'join up' the two possible control paths
again, in this case at line 1250.

The use of flowcharts usually encourages the
introduction of GOTOs as a means of following the
point to point graphical representation of the

program. In general, this use of unconditional
jumps is rather dangerous. If the version of basic
that is being used forces this solution, then a flow
diagram is an excellent method of assessing the
way in which control passes out of the program's
normal succession.

Let's use one last example to examine how the
use of a flowchart allows us to represent
accurately the necessary steps to perform a simple
task: printing out all the numbers between one
and one hundred.

( START ^

N = 1

^ OUTPUT N ^

I

N = N + 1

False

I

N > 100

True

c

STOP

3

10 LET N = 1

20 PRINT N

30LETN = N + 1

40 IF N> 100 THEN END

50 GOTO 20

The use of flow diagrams in this way tends to
encourage a step by step approach to program
writing which, especially in larger projects, often
leads to a rather inelegant result. For those with
even a passing knowledge of the basic language,
the use of a FOR- NEXT loop is obviously indicated.
For example:

10 FOR N = 1 TO 100
20 PRINT N
30 NEXT N
40 END

The flowchart is incapable of representing this
piece of basic 'shorthand', and to follow it exactly
would lead one to a less efficient way of solving
the problem. It does, however, give us some
information on the structure of Sie FOR-NEXT
loop, and so is of value when we come to examine
this and other basic functions, to determine how
they are constructed.

Flow diagrams are particularly useful during
the planning or conceptualising stage of
programming, especially in the 'tricky' parts.
Experienced programmers tend to use them less
than beginners, and will often resort to a flow
diagram to illustrate and document a piece of
software written without their aid. But whether a
flow diagram is drawn out on paper, or it simply
exists inside the programmer's subconscious
mind, the concept of charting the flow of
information and control is central to the use of
computers as a problem-solving tool.

THE HOME COMPUTER COURSE 105

Domestic Science

If you think that there are no
computers in your home, you
ought to take a second look . . .

How many microprocessor chips are there in your
home? There will be one at the heart of your
home computer, of course, but what about the
washing machine, the hi-fi, or perhaps the video?

Everything fi*om the oven to the car's ignition
control and dashboard display can boast the
presence of a chip.

Don't forget the calculator tucked away in the
desk drawer, or your digital watch: the earliest
examples of the mass-produced microprocessor!

Child's Play

Children often make fuller use
of computers in the home than
adults, accepting them as
naturally as the television set.
Knowing about the turtle and
LOGO teaches children to
explore and learn by
themselves. Even a simple
LOGO that only has turtle
graphics can help the younger
child at home

Good quality educational
software is also available (see
page 81). Games can stimulate
and educate, but often the
interests and talents of a child
are best developed and
encouraged by exposure to the
problems encountered in
actually programming a
computer.

LOGO is now becoming
widely and cheaply available for
many home computers, and
offers enormous potential for
encouraging the best approach
to problem solving in many
fields other than computer
programming

The Chip That Cleans

Some washing machines use a
microprocessor to select and
monitor all the various wash
and spin cycles needed to cater
for every machine-washable
fabric. The best combinations
of washing actions and
temperatures, water levels,
rinsing and spin speeds can be
displayed and selected at a
touch. Because there are no
moving parts, except for the
drum of course, the life of the
machine will also be much
longer and servicing costs
considerably reduced

Mobile Micro

Computers are being built in to
cars to improve economy and
provide novel functions. They
can calculate fuel consumption,
monitor speed, act as burglar
alarms and even speak to the
driver, warning of low oil
pressure or battery power

he Sewing Chip

The traditional sewing machine
can produce a beautifully even
and secure stitch, but requires
both skill and patience on the
part of the user.
A microprocessor-controlled
sewing machine can help to
create a complicated
embroidery pattern or an
awkward stitch with little effort.
Besides the selection of pre-set
stitches, these sewing
machines can be programmed
to produce other stitches that
the built-in memory will store,
even when the machine is
switched off

[

106 THE HOME COMPUTER COURSE

No More Burnt Offerings

A computerised oven can help
to produce perfectly cooked
dishes by accurate timing and
temperature control. While the
meal cooks to its programmed
temperature and time, the latest
recipe is on the screen via the
home computer's Teletext
adaptor

Clocking On

Central heating systems can be
more efficiently and fully
controlled by a microprocessor
than by conventional methods.
The electronic clock on the chip
enables different weekday and
weekend heating requirements
to be programmed
appropriately. Separate areas
such as the bedrooms and the
greenhouse or garage can also
have their own programs for
timing and temperature control.
Control such as this saves
energy and cuts the cost of
electricity bills

Video Recorder

In order to maintain the colour,
definition and stability of the
picture, a video tape must be
recorded and played back with
extremely accurate alignment.
The precision control of the
tape mechanism in a VCR is a
task which is well suited to the
powerful microchip found in
some models. It can also take
over the recording of television
programs while you're out. Just
program the automatic tuner for
the times and channels you
want

Your Home Computer

'ou may originally have used
the computer to play games,
but how about something just a
little more challenging?

If you run a business, or if
you are a parent with an interest
in your child's education, you
will probably already be making
use of the computer in many
ways. You could be keeping
records of ail your accounts, or
using the educational value of
the home computer.

But there are a host of other
applications that may never
have occurred to you. If you are
a DIY enthusiast, you may be
interested to note that there is
no limit to the sophistication
that you could build into a
security system, for example.
The computer can monitor
concealed detectors of many
types and initiate alarms,
perhaps even dialling up
emergency services
automatically

Complete Compensation

Microprocessors can provide
very stable control of the colour
television circuitry. They
automatically compensate for .
tuning drift, temperature
variations and ageing of
components. Also, of course,
Teletext services are broadcast
on all four television channels.
The Ceefax service on BBC1
and BBC2 and Oracle on ITV
channels gives you access to a
wealth of miscellaneous
information which is stored on
a computer 'data base'. This is
continuously updated so that
the latest developments in
news, weather, sport and even
stocks and shares are instantly
available at any time

THE HOME COMPUTER COURSE 107

llUil

Insights

The Missing LinIc

Information can be passed from one computer to another over
thousands of miles by means of the modem

Acoiistic Couptef

Most modems in use are 'ail
electronic' devices that connect
directly to the telephone lines,
plugging into the telephone
socket. Telephone companies
set very rigid standards for
devices such as modems. This
tends to make them expensive.
A cheaper solution, since it by-
passes the regulations, is to use
an 'acoustic coupler'. This is a
type of modem that converts the
sine-wave audio-frequency
signals into actual sounds fed
into a small loudspeaker

The term 'modem' is a contraction of 'modulator/
demodulator'. Although modems have been
commercially available for about five years, they
are now being used by owners of home computers
in increasing numbers. If we can all have a
computer of our own, what's the point of
spending money on a modem to link it to the
telephone system?

Modems allow your computer to 'talk' to other
computers all over the world. The only
requirement is for the computer at the other end
of the telephone line to have its own modem. This
other computer may be just an ordinary home
micro owned by another enthusiast, or it could
just as easily be a huge mainframe owned by a
university or financial institution. Connecting
your computer to a large mainfiame can give
access to large databanks, information services
and even 5ie latest stock market prices.
Connecting your micro to a friend's enables you to
exdiange software or send inexpensive
'electronic' mail and even play two-way games.

Modems work in a similar way to the cassette
interface supplied with most home computers.
Both cassette interfaces and modems convert the
computer's ones and zeros into audio fi*equencies.
In the case of cassette interfaces, these fi'equencies
can easily be recorded as thougji they were audio
signals on the cassette tape. With modems, the
audio frequencies are simply sent down the
telephone line to be converted back into binary
numbers by the modem at the other end.

Cassette interfaces, however, need only to
convert binary into audio signals in order to
record on the tape (this process is called
modulating). Or they do the opposite and convert
the audio signals replayed fi*om the cassette into

binary (this is called demodulating). Most
modems, on the other hand, are designed for two-
way communication over a single telephone line
and so they need two fi'equency bands and four
individual fi-equencies. One popular standard
uses a frequency of l,070Hz for a 0 and l,270Hz
for a 1 for transmitting and 2,025Hz for a 0 and
2,225Hz for a 1 for receiving. You will notice that
the two fi'equencies in eadi of the two bands (the
low fi'equency band and the high one) are very
close. There's only a 200Hz difference in
fi-equency for a 1 and a 0 in both bands. This
contrasts sharply with cassette interfaces where
the fi'equency that represents a 1 is usually twice as
high as the fi'equency for a 0. To be able to decode
fi'equencies so close together calls for rather
complex electronic circuitry and this tends to
make modems something of a luxury — modems
can cost as much as many small home micros.

FAX Machine

FAX machines (short for
facsimile machines) are fast
becoming popular in offices in
Europe and the United States. In
Japan even the smallest
businesses have them and
many private homes use them
too. FAX machines can transmit
large documents, including
drawings and pictures, to other
FAX machines in a matter of
seconds, using nothing more
than a built-in modem and an
ordinary telephone

108 THE HOME COMPUTER COURSE

Aiari400&800

Game-playing is the special strength of the Atari range of
computers

The fortunes of Atari, now with headquarters in
Sunnyvale, in California's Silicon Valley, rest on
the phenomenal success of their arcade games.
The first of these was Tong', played in black-and-
white on the television screen.

From this humble beginning. Atari grew, and
eventually became part of the vast Warner
Communications Group, and now, some six years
later, are among the largest manufacturers of
home computers, as well as having a very large
slice of the arcade market.

With its excellent standard of construction,
and reassuringly heavy feel, the Atari range of
home computers, comprising the 800 and 400
models, has set standards which others seek to
emulate. Superb graphics, well-developed
software and — until recently — a very high price
tag all contribute to this quality image.

SelUng for around £150, the 400 differs from
the larger model in price (the 800 costs £280),
in maximum memory size (fixed at 16 Kbytes in
the 400, expandable to 48 Kbytes in the larger
model), in having one instead of two cartridge
ports, and, perhaps of less immediate
importance, being restricted to use via a domestic
television, where the 800 has the option of display
via a monitor. Most obvious, and perhaps most
critical, amongst the differences, however, is in
the keyboard.

In order to overcome the problem of
inconsistent signal levels. Atari home computers
do not use domestic cassette recorders, but rather
require Atari's own model. A large proportion of
users however, have at least one disk drive, in
order to take advantage of the wide range of
software packages available on disk.

Atari Keyboards

The striking difference between
these two Atari models lies in
their keyboards. While the
larger 800 model is equipped
with a full typewriter-
style keyboard the smaller
model has a membrane type
which, while better than some,
still suffers from the in-built
faults of other similar units -
notably a lack of 'feel', and
sometimes unpredictable
response, it is as well to bear in
mind, however, that very
expensive 'ruggedised'
machines, produced for
industrial and military
applications, use this method
to secure against dust and the
occasional spilled cup of
coffee!

THE HOME COMPUTER COURSE 109

Hardware Focus

The Disk Drive

Generally considered to be the most useful peripheral, the
Atari 810 is now beginning to show its age. At only 88 Kbytes
per disk, it's rather small, and since it connects to the
machine via a serial interface it's also fairly slow. However, it
has a sophisticated operating system which has many
features derived from other programs

The Cassette Unit

Being a special unit, designed to work with the Atari
computers, the Atari 410 Cassette Unit is more reliable and
easier to operate than the regular domestic variety. For the
same reason, it doesn't have a speaker, which reduces size
and weight, and neither can an 'ordinary' cassette be used
instead. Shortcomings of the system are principally the lack
of named cassette-files

The Atari Joystidc

The Atari Joystick is one of the poorer add-ons for the
machine. It's a switch-type, so there is no variability. It gives
just a ptfsh in the required direction, and it's rather stiff, so
other makers have produced alternatives

CPU BOARD

Colour Clock

Turning this control will alter
the colour. The various
graphic resolutions are
selected by varying the speed
of this clock

ANTIC

One of the specialised chips
that give the Atari its
impressive features, ANTIC
controls the screen-scrolling,
lightpen and one of the
interrupts

CTIAorGTU

This chip, unique to the Atari,
handles colour, some
miscellaneous I/O and the
Player-Missile graphics

Cotour Adjustment

RAM BOARDS

The Atari can hold up to three
RAM boards. In this way the
memory can be expanded
to 48 Kbytes

Master Clock

KEY

The third of the custom-made
chips, POKEY takes care of
the keyboard, serial I/O, the-
system-timers and also
controls the sound

20 Peripheral Interface
Adaptor

This chip looks after the
Hand Controllers

Speaker

PERSONAUTY
BOARD

The Atari can be made into
quite a different machine by
replacing these ROMs with
others. For example,
alternative languages could
be used. . .

1 10 THE HOME COMPUTER COURSE

MOTHERBOARD

Rectifying Diodes

The Atari power supply gives
out an AC current, so these
components are used to
convert to DC

Peripheral Socket

Power On/Off Switch

Expansion Slots 1,2 and 3

A RAM cartridge can be
plugged into these
connectors, each adding 16K
to the machine

CPU Connector

Since the Atari has the CPU
on a separate card, this slot is
provided to hold it

Left and Right Cartridge Slots

These each take ROM-
cartridges that are pre-
programmed with games or
useful programs

ROM Slot

Video Signal Connector

Interiock Switch

This disconnects the power
whenever the lid is raised, to
reduce the danger to the user

Channel Selector

Power Input
Socket

Monitor
Output
Socket

System Reset
Switch

Keyboard Connector

ATARI 800

Option Switch

PRICE

£280

SIZE

405 X 330 x110mm

WEIGHT

4,200g
6502

CLOCK SPEED

1.79MHz

MEMORY

16 K to 48 Kbytes

VIDEO DISPLAY

Text screen: 40 x 24 characters,
graphics screen maximum of 329
X 192 dots including 16 colours
and 8 shades

INTERFACES

TV connector, monitor, cassette
recorder (dedicated unit), 4
joysticks, serial port

LANGUAGE SUPPLIED

BASIC

OTHER LANGUAGES AVAILABLE

BASIC A+, PILOT, C

COMES WITH

Power supply unit (without plug),
manual

KEYBOARD

57 individual moving keys, plus 3
function keys

DOCUMENTATION

The introductory manuals are
clearly written, accurate and well
produced. Atari will supply full
technical notes for the more
experienced user. This advanced
manual is exactly the same as
used by Atari's engineers, and
not only contains the full circuit
diagram, but also listings of
many of the programs that
control the inside workings of the
computer (the system software).
The only shortcoming of the
manual supplied is the format,
which is in pages for a 3-ring
binder that is not provided

Hand-controller Sockets

Power Indicator UEDs

THE HOME COMPUTER COURSE 1 1

Passwords To Computing

Digital Dialo gue

Input and Output are essential to the operation of any computer
system ■

Analogiie To Digital

In the real world, few pieces of
information come in discrete,
digital steps. Rather, they are
infinitely variable like noise
levels or the tides.

In order to make these data
comprehensible to the
computer, the signal must first
be digitised. The Analogue-to-
Digital (A/D) converter takes
samples from the signal source
at a known, constant rate —
perhaps one hundred every
second. Each of these samples
is stored in a separate memory
location as a digital value, thus
allowing calculations of
variance to be made, and out-
of-limits conditions to be
recognised.

Digital-to-Analogue (D/A)
converters perform a similar
function in reverse, statistical
techniques being used to
smooth out the peaks into a
regular curve

Input/Output, or I/O as it is commonly
abbreviated, is the term used to describe the
transfer of information between the CPU (the
Central Processing Unit forming the heart of the
computer) and the 'outside world'. The 'outside
world' in this context means any devices that may
be connected to the computer. It does not include
RAM and ROM memory, which are considered
to be integral to the computer. The distinction
between what is held to be 'inside' the computer
and 'outside' it is somewhat arbitrary. But all of
the logic circuits (see page 92), designed to work
in close conjunction with the CPU and main
memory, are considered to be part of the 'inside'
of the computer.

External devices, which use I/O for
communication with the computer, include a
wide variety of peripherals ranging from the
keyboard to floppy disk drives, joysticks, printers
and video display units.

When the CPU wants to retrieve data from
memory, it has first to 'address' the location where
the byte of data is stored. Similarly, if the CPU
wants to store a byte of data for later use, it must

first address the location where the item of data is
to be stored. This process is called 'memory
addressing'. It involves the CPU putting the
binary digits corresponding to the desired
memory location on a set of 16 wires connected to
the CPU's 'address pins'. These wires are called
the 'address bus'. Special circuitry in the memory
section is able to decode these 16 binary digits to
select the correct memory location. (Sixteen
binary digits can give 65,536 unique
combinations of ones and zeros and can therefore

address that many different memory locations.)

If the computer wants to communicate with an
external device, it also has to 'locate' that device in
a similar way. Only eight address lines are
available. This limits the total number of separate
I/O locations that can be selected to 256. This is a
small number compared with the addressing
power of 16 address lines, but in practice 256 is
more than adequate. There's usually no need for
huge numbers of external devices to be connected
to a computer.

Selecting Devices

To find out how the computer actually selects an
external device and sends data to it, let's consider
one of the simplest output devices possible — an
LED (Light Emitting Diode) mounted on the
keyboard of the computer to show when the 'caps
lock' key has been pressed (there's a key and an
LED like this on the BBC Microcomputer). To
the computer, the LED is simply another external
device to which it can send data. In the case of a
single LED, the data will be either a single 1 (to
turn the LED on) or a single 0 (to turn the LED
off). Even though it is just a humble LED
requiring a single bit of data, it still needs to have
an address or location. The CPU can't spend its
whole time addressing one LED. It needs to be
able to select the LED once only to tell it when it
needs to switch on, and again to tell it when to
switch off. Suppose, for the sake of argument, the
LED has an I/O address of 32. To select it, the
address lines will have to be set by the CPU to the
binary equivalent of 32. This is 00100000 in
binary. TTie LED wiQ have a special 'decoder'
circuit that will ignore all other combinations of
bits on the address lines. When the address line
becomes 00100000, this decoder circuit
recognises it and produces a high voltage and
therefore a 'true' output. The next part of the
circuit needed to switch on the LED is a small
chip called a 'data latch'. This latches or holds the
data sent to it so that the LED stays on or off until
the next time it is addressed and new data is sent
to it. This process is known as 'toggling'.

Most of the external devices with which the
computer communicates are considerably more
complex than a single LED. A printer is a typical
peripheral and each time the computer
communicates with it, the data transmitted will
represent the code for a whole character to be
printed. Usually, when large amounts of

1 12 THE HOME COMPUTER COURSE

Passwords To Computing

Buffer

Input

information are to be transferred, as for a printer,
a special I/O interface chip is used. Such chips
simplify the task of the computer engineer,
because the interface circuit is designed to
incorporate most of the circuitry needed into one
diip. One of the most popular of these is the 8255
PPI (Programmable Peripheral Interface). This
40-pin diip contains three eight-bit I/O ports.
That means there are 24 I/O pins on the chip,
eight pins each for I/O ports A, B and C. Each of
these ports can send eight bits (one byte's worth)
of data at a time to a peripheral device such as a
printer, or receive eight bits of data at a time from
an input device such as a keyboard.

To send eight bits of data to a printer, the CPU
will first address the PPI and then send it the eight
bits of data on the data bus. This data will be
stored in a temporary one-byte memory cell

• 1
1 I

1
1
1

■ 1

1 —

AdifrBSS

! 1
• 1

I

Lilies a

S 1
i 1
• ■

1

Address

1

1
1
1

Decoder

1
1

1

1
1

1

1

1

1
1

1
1

1
1
1

1
1

Input/Output

In the simplest control
applications, such as that
illustrated, the CPU handles
only a single piece of
information, whether or not a
switch has been pressed. The
buffer — itself a short-term
memory — simply holds the
data until the CPU next 'polls'
the device in question. The
address decoder indicates the

source of each signal, and
when a change of state is
recognised, i.e.. that the switch
has been pressed, the CPU
delivers an appropriate
response, in this case changing
the clock display from actual
time to the time at which the
VCR's auto timer will turn on
the recorder. Within the output
stage, the same procedure
works in reverse

CPU

1 — —

Address
Lines

Video Recorder

Address Latch
Decoder

Dutpiit

The CPU stops running the program it is
executing periodically and takes a quick look at
all the input ports. K it finds data there waiting to
be input, it instructs the port to put the data on the
data bus. The process of enquiry of the input
devices is known as 'polling'.
The other method uses 'interrupts'. The device

within the chip, called a register. The PPI will then
make this data available on the appropriate set of
I/O pins. A similar principle, but working in
reverse, allows data fi"om external input devices to
be stored in a register in the diip, and then put
onto the data bus when the CPU sends it the
appropriate signal. As noted above, external
devices cannot be allowed to put their data onto
the computer's data bus continuously — it is
needed to transfer data to and from memory and
by other I/O devices. The I/O chip stores the
data temporarily and only puts this data on the
data bus (to be picked up by the CPU) when the
CPU tells it to do so.

How does the CPU know if an external device
is trying to send data to the computer? Briefly,
there are two main techniques that can be used.

Serial And ParaM Ports

Most modern microcomputers
provide both serial and parallel
ports, the former passing data a
bit at a time, the latter in whole
bytes. The most common type of
serial convention, known as
RS232C, uses either a 'D-type'
sub-miniature connector, a 25-
pin example of which is
illustrated here (left), or more
rarely a DIN plug like those used
in hi-fi systems.

The parallel port (right)
follows the IEEE488 convention,
developed by Hewlett Packard
and adopted as an industry
standard by the Institute of
Electrical and Electronics
Engineers in the USA

wanting attention sends an interrupt signal
directly to the CPU and this forces the program
being executed to stop while the input port is
attended to. The advantages and disadvantages of
these two methods will be described in more
detail later in the course.

The I/O we have described so far is called
'parallel I/O' because data is input or output one
byte at a time using eight I/O wires or lines (eight
bits in parallel). Another technique is called
'serial I/O'. Here the information in each byte is
fed in or out a bit at a time, one bit after the other.
Some printers use serial interfaces, and the output
from modems (see page 108) is also in serial form.
The main advantage is that, essentially, serial
communication allows a single pair of wires to be
used instead of eight or more.

THE HOME COMPUTER COURSE 113

Flat Spin

Magnetic disks spin at higli speed, within disk di
information that can be 'read' by your computer

mmmm

Home computers will 'forget' everything you
have programmed them with once the power is
switched off. At best this can be a minor irritation,
at worst a major disaster as an entire evening's
progranmiing disappears for good. For this very
reason, the makers of home computers
incorporate a method by which the contents of
the computer's memory can be permanently
stored. TTiis usually takes the form of a cassette
tape on which the program is stored digitally as a
series of tones (see page 94).

However, when dealing with long programs, or
a collection of small programs that need to be
frequently used, the time taken to find and load
the program from a cassette can be a major
setback. There are two reasons for this. This first is
that a tape must be started at the beginning in
order to locate a program recorded on it -
although cassette recorders with tape counters
greatly assist here. ^

The second cause of the problem is the way in
which the program is stored. The patterns of bits
held in the memory have to be converted into a
corresponding sequence of tones: a high tone
represents a bit that is on (or set to one), and the
lower tone represents a bit that is off (or set to
zero). These tones must then be recorded onto the
cassette tape. The fastest practical rate at which
this transfer can occur is 150 bytes a second. Any

PROTECTIVE ENVELOPE

PROTECT/PERMIT SLOT

SECTOR

REGISTRATION HOLE

TRACK

The Ftoppy Disk

The surface of a disk is divided
up into a number of separate
bands called tracks. These
tracks are further subdivided
into sectors. On the Apple II,
for example, each track is
divided into 16 sectors. Each
sector has an address field and
a data field.

The Disk Operating System
accesses the individual sectors
on a track by using the address
field, which contains the track
and the sector numbers, and an
identifier (to check that the user
is reading the right disk). Thus
it can retrieve information in
much the same way as it is
retrieved from a memory
location (by using its address)

ACCESS SLOT

faster, and the possibility of errors increases to the
point where the system fails to be reliable.

A conventional cassette system using C-10
tape can take as long as five minutes on each side
to find and locate a program. This is assuming
that a fast loading system is being used. Some
systems work as slowly as 30 bytes a second. For
those long programs you really need a recording
system that will find the beginning of the program
and load it in a matter of seconds.

Sudi a storage system is the floppy disk and it
can be used on most of today's home computers.
If you imagine the yards of tape stored inside a
cassette tape laid out in the form of a spinning
disk some five inches across, you will appreciate
how quickly any information stored on the disk
can be located. This disk is placed inside a
protective envelope and slotted inside a disk
drive.

The drive's function is to spin the disk (inside its
envelope) at a constant speed, and to provide a
means of transferring programs on and off the
disk fi-om the computer. It does this through a
recording and playback head, similar to that on a
cassette recorder, but very much smaller. This
head can move backwards and forwards across
the surface of the spinning disk, unlike the
cassette, which can only move the tape past the
head.

Analogue Board

This circuitry converts the
signals coming from or going
to the head. It translates the
digital form used in the
machine to the analogue form
that goes on the disk

Indicator

This Light Emitting Diode
shows whether access is being
made to the disk drive

Driving Hub

This engages with the plastic
disk and spins it round inside
the envelope

Care Of Your Disk

Floppy disks are delicate and
should be handled with respect.
Follow the manufacturer's
recommendations carefully

OONTBEND!

DONT STACK!

KEB> AWAY FROM MAGNETS

STORE CARffUUY

KEEP AT ROOIM TOMPKATURE

1 14 THE HOME COMPUTER COURSE

Insights Kuul

Ribbon Cable Connector

This provides secure, yet
detachable, connection of tiie
ribbon cable

Ribbon Cable

Information iS transferred to
and from the disk drive by way
of the ribbon cable. It contains
the eight-bit data path, and
other control signals

Driver Motor

This spins the driving hub

Read/Write Head

Stepper Motor And Drive Screw

A very accurate electric motor
that moves the head across the
surface of the disk

Cantilever/Loading Mechanism

Connected to the door flap, this
lever mechanism ensures the
disk's precise location on the
driving hub

3

o
o

Read/Write Head

This is a highly magnified
picture of the head that reads
and writes data to the surface
of the disk. It is similar to the
head on a cassette recorder, but
almost invisible to the naked
eye.

Unlike a tape that is just one long string of
bytes, a disk is 'formatted' in a series of concentric
circles, each of which is treated by the system as
small chunks, usually 256 bytes each. Each of
these 'sectors' has an address.

When a program is to be written to the disk, the
first thing that happens is that the head is moved
to the directory, a special file which acts as an
index to the whole disk. This is examined to find
out where to put the file. If it's being re-written,
the first sector of the old copy is found, and the
new data is stored starting there. A new file won't
have an entry in the directory, so one must be
made, then the first empty sector is filled with the
data, with more sectors being filled as required.

The advantages of high-speed efficiency and
large storage capacity offered by the disk explain

the substantial difference in price between the two
systems. Disk drives sell from about £120,
whereas a cassette player is usually less than £20.

The most important factor in this price
difference is the precise engineering that is
required. The recording and playback head of a
disk drive is almost invisible and must be placed
to within hundredths of an inch.

The mechanism that moves this minute head is
based on an electric motor, which can turn by
fractions of a degree. This is coupled to a shaft
that carries the head and moves it across the
surface of the disk in minutely calculated steps.
To ensure that the disk spins at a constant speed,
complex el6ctronics are used and all the
components are mounted on a rugged die-cast
frame to reduce the effects of heat and vibration.

THE HOME COMPUTER COURSE 1 1 5

Basic Programming

Braving The Elements

Subscripted variables, unlike their simple counterparts, can contain
any number of elements

In our earlier program for calculating the number
of days to Christmas we encountered a new type
of variable called a 'subscripted' variable. These
differ from ordinary or 'simple' variables in that
they can have any number of compartments or
elements within the box. Simple variables
recognise two letters or letters followed by
a digit from 0 to 9 (some versions of basic allow
whole words to be used as variable names). A, B,
B1, C3 and R2 are all simple variables. Subscripted
variables look like this: A(6). B(12) or X(20). The
subscript is the number in brackets. The examples
we have given would be read as: A sub six', 'B sub
twelve' and 'X sub twenty'.

If we think of a simple variable as being a box
with a name or label on it, we can think of a
subscripted variable as a box containing a
specified number of internal elements. If we want
a variable with 12 elements, we create it initially
using the basic DIM statement, like this: DIM A(12).
Any letter of the alphabet may be used.

Assigning values to simple variables is
straightforward, using either LET or INPUT
statements, like LET A = 35, LETB1 = 365 or INPUT C3.
Values can be entered in the elements of a
subscripted variable in the same way. Let's see
how we would assign values to a subscripted array.
(Array' is an alternative name for a set of
subscripted variables.) For example:

10DIMA(5)

creates a subscripted variable with five elements.
We can now assign a value to each element:

20 LETA(1) = 5
30 LET A(2) = 10
40 LET A(3) = 15
50LETA(4) = 20
60LETA(5) = 100

To find out how these variables differ from simple
variables, let's assign values to a few simple
variables:

70LETX = 5
80LETY = 6
90LETZ = 7

Try entering all these on your computer and then
check the contents of each variable using the
PRINT command. Many of the statements in basic
also function as commands. After you have
entered the statements above, check them by
LISTing them and then type RUN. Now you can
type PRINT X<CR>. You should see 5 instantly

displayed on the screen. Next type PRINT Y. The
computer will respond to this PRINT command by
displaying 6 on the screen. If you want to check
the elements in the subscripted variable, type
PRINT A(1 ) to find out the value of the first element
in the array. The computer should respond by
printing 5 on the screen. Try PRINTing the values
of A(3) and A(5).

The important difference between subscripted
variables and ordinary variables is that the
subscript can itself be a variable. To see what this
means, type PRINT A(X). The screen will respond
with the figure 100. Why?

Look at the list you have typed in and check the
value of variable X. It is 5. A(X) is equivalent to A
(the value of variable X) and this is equivalent to
A(5). Typing PRINT A(X) is therefore exactly
equivalent to typing PRINT A(5). What value would
you expect if you typed PRINT A(Y — X)? Before
actually trying it, see if you can work out the
answer.

Assigning Values

If there are only a few simple variables, the LET
statement is the simplest way of assigning values
to them. Subscripted variables may well have a
large number of elements in the array, so let's see
what the alternative methods of entering the
values are:

10DIM A(5)

20 PRINT "INPUT THE VARIABLES"

30 INPUT A(1)

40 INPUT A(2)

50 INPUT A(3)

60 INPUT A(4)

70 INPUT A(5)

This method is just as tiresome to type in as using
LET statements, though it would certainly work. If
we know exactly how many variables there are (in
this case there are five) it is easier to use a FOR-
N EXT loop, like this:

10DIMA(5)
20FORX = 1T0 5
30 INPUT A(X)
40 NEXT X

This program would expect five values to be typed
on the computer keyboard when the program was
run. The RETURN key would have to be pressed
after each figure had been entered. If we know
beforehand what the values in the variable are, it is

1 16 THE HOME COMPUTER COURSE

easier to enter them using a READ statement
together with a DATA statement, like this:

10 DIM A(5)
20FORX = 1T05
30READA(X)
40 NEXT X

50 DATA 5, 10, 15. 20, 100

Try this short program, and then test the contents
of the array using the PRINT command (that is, use
PRINT after the program has been RUN. For
example, PRINT A(1)<CR> and PRINT A(5). Now we
can add a few lines to the program to print the
elements in the array for us automatically:

60FORL = 1T0 5
70 PRINT A(L)
80 NEXT L
90 END .

RUN this program and check that the correct;
values are printed on the screen. Then retype line
50 using five different DATA items. Remember that
the numbers in a DATA statement must be
separated from each other using commas, but
there must be no comma before the first number
or after the last one.

The simplest way to assign values is to use READ
and DATA statements. If the values will be different
every time the program is run, using the INPUT
statement inside a FOR-NEXT loop is probably the
best way. If the total number of elements in the
array is fixed, the number can be used as the
upper limit in the FOR statement.

Let's use all we have learnt so far to build a
short but powerful program. Suppose we wanted
to sort some numbers into ascending order.
Before setting out to write the program, the first
thing to do is to figure out how to solve the
problem in a logical way. When the way to solve
the problem seems clear, write down the steps one
after the other using clear, short English
sentences.

Suppose we start with five numbers: 4, 9, 2, 8,
3. Sorting these into ascending order is a trivial
problem. We just scan along the line and notice
which is the smallest, and put it on the left, and
then repeat the process for the remaining digits.

The computer, however, needs a very precise
set of instructions, so we shall have to think very
clearly about what steps are required. Here's one
approach: Compare the first digit with the second
digit. If the first digit is bigger than the second one,
swap them. If the first digit is smaller than the
second one, leave their positions unchanged.

Compare the second digit with the third digit. If
the second digit is smaller than the third one, leave
their position unchanged.

Repeat the process of comparing pairs of digits
until the last pair of digits has been compared.

If there were no swaps, all the numbers must be
in order. If there were any swaps, go back to the
beginning and repeat the process.

If you think about this process, you will see that
it will indeed sort any group of numbers into

Basic Programming

ascending numeric order. Look at what would
happen to our original set of numbers as each pair
of digits is compared:

4
4
4
4

9 2

2 9

2 8

2 8

8 3

8 3

9 3
3 9

All the pairs have now been compared and
swapped where necessary. Since at least one swap
took place, go back to the beginning and repeat
the process:

4
2
2
2

2
4
4
4

8 3

8 3

3 8

3 8

9
9
9
9

There were still swaps, so go back to the beginning
and repeat:

2 4 3 8 9
2 3 4 8 9
2 3 4 8 9

There were no swaps, last time through, so every
number must be smaller than the number to its
right. The numbers must be in ascending order
and the operation can be terminated.

Using subscripted variables allows a sort
routine like this to be implemented easily in basic,
because the subscript itself can be a variable. If
our original five numbers were the values in an
array; so that A(1) = 4. A(2) = 9. A(3) = 2, A(4) =8 and

A(X + Y-Z)

X

■

A(5 + 6-7)
= A(4)

Subscripted
Variables

Subscripted variables
(variables with several
'compartments' in the box)
increase the power of BASIC
enormously. Here, variable A
has the subscript X + Y - Z.
Each of these is a variable,
and the value of each is
shown inside the small
boxes. X has the value 5, Y is
6andZis7. X + Y-Zis
therefore equivalent to 5 + 6 -
7, which is 4. A{4) is the
fourth element in the array. Its
value is 20. PRINT A(X + Y-
Z) will therefore result in 20
being printed on the screen

yj 5 -r- 10;^ 15/- 20 -HOO

A(1)

m

A(3)

m

20

THE HOME COMPUTER COURSE 1 17

Basic Programming

A(5) = 3, then if X has the value 1, A(X) wiU be the
contents of A(1), which is 4. A(X + 1) will be the
contents of A(2), which is 9, and so on.

Lx)ok at the program and see if you can see
exactly what is going on. Line 20 sets variable N to
the number of numbers we want to sort. Let's
assume we want to soit five numbers: when the
program is run we will type in 5 and then hit
RETURN.

Line 30 is the DIMension statement. If N is 5, it
sets the size of the array to 5. This line is
equivalent to DIM A(5).

Lines 40 to 60 are a FOR-NEXT loop that
allows us to type in the five numbers. Most
versions of basic prompt the user with a question
mark on the screen. RETURN will have to be
pressed after each number has been entered. The
numbers may be more than one digit, and may
include decimal fractions.

Line 90 sets the variable S to 0. This variable is
being used as a 'flag'. Later in the program, A is
tested to see if it is 1 or not. It is only ever set to 1 if
two numbers have been swapped, as we shall see
in line 240. We shall investigate the use of 'flags' in
more detail later in the course.

Line 100 sets up the limits for a loop; in this
case from 1 to 4 (because N is 5 so N — 1 is 4). The
first time through the loop, L is 1 so A(L) in line 110
will be A(1 ) or the first element in the array and A(L
+ 1) will be A(2), the second element in the array.
The next time round the loop, L will be
incremented to 2, so A(L) will be equivalent to A(2)
and A(L + 1) will be equivalent to A(3). Line 110
tests to see if A(L) is greater than the number
immediately to its right in the array. The sign for
'greater than' is >.

If the first number is bigger than the next one,
the program branches to a subroutine that swaps
the numbers. If the first number is not bigger than
the next one, there is no branch to the subroutine
and BASIC simply continues to the next line, which
is the NEXT L statement. After the loop has been
repeated four times, it stops the program and goes
to line 130 which tests the 'swap' flag, S, to see if it
has been set or not. If it has been set (in the 'swap'
subroutine), the program branches back to line 90
to repeat the comparison process. If S is not 1 it
means no swap took place, so all the numbers are

Basic Flavours

THEN

If this program is to be run on the ZX81 or
Spectrum, line 130 must be amended to
read: 130 IF 8=1 THEN GOTO 90

This statement is not available on the
ZX81 and Spectrum, so use STOP instead

LET

In assignment statements, such as
90 LET S= 0, the word LET is optional on
most machines but not on the ZX81 and
Spectrum. The Lynx adds it into program
listings automatically

in order. The rest of the program simply prints
them out.

The swap subroutine needs a temporary
variable to store one of the numbers to be
swapped. After the two numbers have been
swapped in lines 210, 220 and 230, the 'swap' flag
S is set to 1 and then the program RETURNS to the
main program.

10 PRINT "HOW MANY NUMBERS DO YOU WANT

TO SORT?"

20 INPUT N

30 DIM A(N)

40 FOR X = 1 TO N

45 PRINT "NEXT NUMBER"

50 INPUT A(X)

60 NEXT X

70 REM

80 REM SORT ROUTINE

90LETS = 0 ' ^

100FORL = 1TON-1

110IFA(L) >A(L + 1) THEN GOSUB 200

120 NEXT L

130IFS = 1 THEN90

140FORX = 1TON

150 PRINT "Ar;X;") = ";A(X)

160 NEXT X

170 END

180 REM

190 REM

200 REM SWAP SUBROUTINE

210 LET T = A(L)

220 LETA(L) = A(L + 1)

230LETA(L + 1). = T

240 LET S =1

250 RETURN

Exercises

■ Extend the program to find the average value
of the numbers input. The average is equal to the
sum of items divided by the total number of items.
The simplest way to do this is to insert a GOSUB
just before the END statement in line 170. The
subroutine should read each of the elements in the
array and add the values to a *sum' variable. After
all the elements have been added, the sum should
be divided by the number of elements. The sum is
most easily derived by using the number of
elements as the upper limit of a FOR-NEXT loop.

■ Change one line in the program so that the
numbers will be sorted in descending order.

■ This exercise is directed mainly at owners of the
TI99/04A which does not like having variables
used as subscripts in subscripted variables. TI
BASIC does, however, accept statements such as
DIM A(12). Rewrite the program so that the INPUT
statement expects an exact number of numbers to
be input, 12, say. This will avoid the problem of
using a variable name as a subscript. Lines 100
and 110 will also have to be changed. The swap
subroutine will not work in TI basic for the same
reason. This will have to be changed too.

■ A tough one. Our way of sorting numbers is by
no means the only way to do it. See if you can
think up an alternative method.

8 THE HOME COMPUTER COURSE

Two's Company

Although computers give speedy answers to complex arithmetical
problems, they deal with them in the simplest way

In the last part of our course on binary, we
discovered how computers could add. Now we
look at the process of multipUcation.

If you had to multiply 14 by 12, a simple way
would be to do a multiple addition, for
example 14+14+14+14+ ... (12 times).
Since multiplication is in one sense a form of
repeated addition, this approach would
certainly work, and is how the first computers
dealt with multipUcation. However, the
method is awkward and time-consuming, so
computer designers evolved a more efficient
method.

When two numbers are multiplied, the
operation is usually written down on paper like
this:

14

x12

28

+14

168

(a final 0 is often written
to keep the digits in
the correct column)

Exactly the same process works in any base of
numbers. Let's look at an example in base two or
binary:

101

x11 .

101
+ 101
1111

With larger numbers the method is exactly the
same, so let's go back to the example of 14 X 12
and do it in binary:

1110
xllOO

0000
0000
1110
1110

(14)
(12)

10101000 (168)

Multiplication is even simpler in binary than in
decimal because there can never be a carried
digit. When you multiply a number by 1 the
number is unchanged, 14 X 1 = 14 and when you
multiply a number by 0 the answer is 0, 14 X 0 =
0. This is true in binary, decimal and all number
systems.

When mathematicians looked at similar
calculations to the one above they saw a simple

37X15

i

100101 X 1111

i

10 0 10 11*
10 0 10 1
10 0 10 1
10 0 10 1

110 0 0 1 0 1 oil

THE NUMBER (37) . . .

SHIFTED ONE PLACE

SHIFED TWO PLACES
SHIFTED THREE PLACES

RESULT

CO

pattern emerging: binary multiplication consists
of only two operations, 'shifting' and addition.
And this is exactly how the computer does a
multiplication. First it shifts 'copies' of the upper
line into their correct position (determined by the
Is and Os in the lower line) and then it adds all the
'copies' together.

The computer needs to have a large digit
capacity to perform multiplications. When the 4-
digit number 1110 was multiplied by the 4-digit
number 1100 the answer contained 8 digits
(10101000) and in general the result of a
multiplication can be up to twice the length of the
largest number.

It may come as a surprise to learn that a
computer's multiplication can return an incorrect
result. Nearly all these errors can be traced back
to the amount of space the machine's designer has
allowed to hold the answer. If insufficient space
has been allocated, 'overflow' will occur, the least
significant digits will be lost, and the result will
thus be erroneous.

Shifting Into Multiplication

Multiplication is much easier in
binary than it is in decimal. The
same process of long
multiplication used in the
decimal system is applied, but
because there are only two
numbers involved in the
multiplying (0 and 1), the
operation is very simple. When
a number is multiplied by 1 the
result is obviously the same
number. In the illustration,
where 100101 (37) is multiplied
by 1111 (15), four copies of
100101 appear. Each copy is
shifted to the left according to
the position of the 1 that
multiplies it. Finally all the
copies are added together to
give the answer 1000101011
(555)

Gottfried Wilhelm Leibniz

Leibniz (1646-1716) was a
contemporary of Isaac Newton
and made contributions to
mathematics, science and
philosophy. He invented a
machine that could multiply
and divide. He also investigated
the possibilities for using
binary arithmetic in calculating
devices, though the first known
reference to binary numbers
was by Francis Bacon in 1623.
In his later years he fell into
dispute with Newton over the
invention of the calculus

THE HOME COMPUTER COURSE 1 19

Pioneers In Computing

Sir Clive S

The entrepreneur/engineer who made the computer more
widely available

Stop someone in the street and ask them if they
can describe what a Spectrum or a ZX81 looks
like and you'll probably get a blank stare. But ask
if they have heard of Sir Clive Sinclair and the
chances are anybody who reads newspapers and
watches television will know that you are talking
about a millionaire electronics genius.

It is fair to say that this 43-year-old ex-technical
journalist is the most famous individual in the
computer world. He has been described as doing
for the personal computer what Henry Ford did
for the motor car and Freddy Laker achieved in
the air travel industry.

His success at designing and marketing the
world's most successful computer — the ZX81 —
was rewarded earlier this year when he was
awarded a knighthood for putting Britain back in
the technological race against Japan and the USA.

Clive Sinclair was bom in London in 1940. He
left school at 17 after completing his education at
St George's College in Weybridge. He was a
technical journalist for four years before forming
his first company, Sinclair Radionics, in 1962. The
first products were radio and amplifier kits sold by
mailorder.

Sir Clive has made his fortune from his
ingenious computer designs that have turned into
the ZX81 and ZX Spectrum. But he was also
responsible for the world's first low-cost,
electronic calculator which he launched in 1972.
There was even a gold plated model which sold

for £2,750! He also made one of the first digital
wristwatches using a microchip.

His most recent innovation is the flat-screen
television which is no bigger than a paper-back
book and which has a two-inch screen. This will
seU for £80 and will be available later this year.

Now Sir Clive is working on his most ambitious
project of all — an electrically-powered car. The
team working on this project is working towards
building a new electric vehicle for town use.

But Sir Clive is also an established publisher.
He has written around 17 books himself on
electronic subjects and in 1981 launched a new
publishing company called Sinclair Browne. He
publishes around 20 fiction and non-fiction books
a year.

Early in 1983 the Guardian newspaper named
him 'Young Businessman Of The Year'. The
influential trade magazine Computing gave him
the tifle 'Computing's Person Of The Decade'.

He escapes from the world of electronics by
going to the theatre and poetry readings and he is
a trustee of the Cambridge Symphony Orchestra.
Sir Clive divides his time between his Cambridge
headquarters and his London office and his silver
Porsche 924 Carrera can often be seen streaking
along the Al.

Sir Clive is a great patriot. His aim is to
persuade fellow Britons that there is nothing that
the American and Japanese can do that can't be
done in their own country.

1962

Clive Sinclair forms Sinclair
Radionics in Islington,
London, to sell radio and
amplifier kits by mail order

1972

Sinclair produces world's first
pocket calculator, the
Executive, which sells for £79
and earns over £2.5 million in
export revenue

1975

Sinclair launches one of the
first digital watches, which he
calls the Black Watch.
However, the company
sustains losses due to
difficulties with chip supplies

1976

National Enterprise Board
gives Sinclair cash to develop
his pocket television which is
put onto the market the
following year after a 12-year
development programme

1979

Sinclair sets up a new
company, Sinclair Research,
to develop products in the
consumer electronics field

1980

The new company launches
its first product, the ZX80
computer, which is acclaimed
as the first computer to sell
for less than £100

1981

Sinclair developes the ZX81
computer, which wins a
Design Council Award and
sells more than one million
units in two years

1982

The Spectrum is introduced
to sell alongside the ZX81 but
designed for a wider range of
home, office, and educational
uses

1983

The long-awaited Microdrive
arrives, along with the
Interface I and Interface II,
which expand the Spectrum
to take ROM cartridges and
local area network facilities.
Sinclair also announces his
new flat-screen personal
television set after a four-year
£4 million development
programme

1984

Sinclair launches the QL,
aimed at the small business
market, with 128 Kbytes of
RAM and 200 Kbytes of back-
up in two built-in Microdrives

120 THE HOME COMPUTER COURSE

Home Computer News

The Personal Computer World Show, held last
year at the end of September, has become the
leading venue for the unveiling of new computer
products, and the exchange of ideas between
computer owners, clubs and companies.
Amongst the crowds of schoolchildren eager to
demonstrate their prowess at every new arcade
game, bewildered businessmen trying to decide
which micro offers the best value, and the
cacophony of explosions and dalek-like voice
synthesisers, were to be seen a vast array of new
computers, add-ons and software. Some were
available for sale, though an unfortunate number
appeared to be little more than manufacturer's
prototypes.

Spectrum Interface ^

Plug-in cartridge software is
now available for the Sinclair
Spectrum. But you will need to
buy a special interface, which
can also connect with joysticks

fPORTABlLITY WITH
COMMUNICATIONS

*16-BIT PORTABLE

COMPUTER
★128K byte RAM
★EXPANDABLE TO 258k

fr128K byte BUBBLE
MEMORY /

*8 LINE LIQUID^ ^
CRYSTAL piSP 4Y
★FULLCpMMUf .
★OPTIONAL FUr H'
WIDTH PRINTE >
*0*^tONAL PL' f
MSK DRIVES I
_5Kf i

^ New From Sharp

The Sharp PC-5000 is a truly
portable computer complete
with an eight- line Liquid
Crystal Display and an optional
built-in silent printer. It uses
'Bubble Memory', which has
approximately the same
capacity for storage as a disk
drive, but no moving parts

Flight Simulators

The number of games programs
and specialist games software
companies is growing each
month. Several produce Flight
Simulators - which are both
educational and fun

Talking Back

'My Talking Computer' from
Electroplay is designed
specifically for teaching young
children, and features a talking
clock and other educational
features^

■POT UAHiTV

THE HOA/IE

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THE LAST WORD IN LOGIC